Breast cancer remains an important threat to women's health. X-ray mammography currently is the most widely used breast imaging tool for early detection, and is the modality approved by the Food and Drug Administration to screen for breast cancer in women who do not show symptoms of breast disease. A typical mammography system takes a projection image of the compressed breast, using a collimated x-ray source at one side and a film/screen unit at the other side of the breast. In the United States, typically two views are taken of each breast, one from above (cranial-caudal, or CC) and one from the side (mediolateral-oblique, or MLO). The x-ray source is an x-ray tube typically operating at 25-40 kVp, using a molybdenum or rhodium rotating anode with a focal spot of about 0.3-0.4 mm and, in some cases, 0.1 mm or less. An anti-scatter grid between the breast and the film/screen unit reduces the effects of x-ray scatter. The screen converts x-ray energy to visible light to which the film is exposed to record the image. In each view, the breast is compressed to reduce patient motion and also for reasons such as reducing scatter, separating overlapping structures in the breast, making the thickness of the imaged breast more uniform, and providing more uniform x-ray exposure. Currently, flat panel array receptors are replacing the film/screen units in mammography systems. The Selenia™ digital mammography system with such a flat panel x-ray receptor is offered by Lorad, a subsidiary of the assignee Hologic, Inc. of Bedford, Mass. Digital mammography has significant advantages and in time may fully supplant film/screen systems. Additional information regarding digital mammography systems and processes offered by the common assignee can be found at <www.hologic.com>.
Mammograms, whether from film/screen units or from digital systems, are particularly difficult to read, and the rate of false negatives and false positives is significant. Many advances have been made in recent years in image acquisition and in image processing, but a need still remains to reduce the rates of false negatives and positives, at least in screening mammography. Additional information can be gained through modalities such as CT and MRI, but examination and interpretation time and cost and other factors have limited their use in screening for breast cancer. Ultrasound breast examination has been proposed as an adjunct to x-ray examination, with synthesized ultrasound images of thick slices of the breast as they would appear in the same projection view as an x-ray view displayed together with the x-ray view, and a unit taking both x-ray and ultrasound images has been proposed. See, e.g., Patent Application Publication No. U.S. 2003/0007598 A1 and U.S. Pat. No. 5,983,123. Digital tomosynthesis has been proposed for x-ray breast imaging, and a laboratory unit is believed to have been installed at the Massachusetts General Hospital (more than a year before the filing date hereof), as reported in Wu, Tao, 2002, Three-Dimensional Mammography Reconstruction Using Low Dose Projection Images, PhD thesis, Brandeis University, incorporated here by reference. See, also, Patent Application Publication No. 2001/0038681 A1 and PCT application International Publication No. WO 03/020114 A2 published Mar. 13, 2003, both incorporated herein by reference. Digital tomosynthesis in more general contexts also has been proposed. See, e.g., U.S. Pat. Nos. 6,289,235 and 5,051,904, commonly assigned U.S. Pat. No. 4,496,557, and Digital Clinical Reports, Tomosynthesis. GE Brochure 98-5493, 11/98, all incorporated herein by reference. Reference markers can be used in x-ray imaging for purposes such as checking the rotation angle and unwanted shift of center of rotation of an x-ray source and receptor, and fiducial phantoms can be used in 3D angiography to calibrate for irregular scan geometries. See, e.g., U.S. Pat. Nos. 5,051,904, 5,359,637, and 6,289,235, N. Navab, et al., Dynamic geometrical calibration for 3D cerebral angiography, SPIE Vol. 2708, pp. 361-370, and said PCT published application WO 03/020114 A2, all incorporated by reference here.
It is believed that no breast tomosynthesis systems are commercially available currently for clinical use in breast imaging, and that improvements in x-ray imaging and tomosynthesis are a desired goal. Accordingly, it is believed that a need remains for improved and practical tomosynthesis mammography.